FIG. 1 shows an example dot array of a matrix type OLED panel in which three dots, namely the typical R (red), G (green), and B (blue) dots, form one color pixel. FIG. 2 shows an example dot array in which, in addition to the RGB dots, W (white) dots are also provided.
The RGBW type display apparatuses, which additionally include W dots, consume less power and are brighter because the W dots have higher emission efficiency than RGB dots.
Methods for realizing such RGBW panels can be divided into types: methods which employ OLED emitting each of RGBW colors provided for each dot; and methods which employ white OLEDs and RGB optical filters (color filters) superposed on dots other than W dots, as described in Japanese Patent Laid-Open Publication No. 2003-178875, for example.
FIG. 3, which shows a CIE (Commision Internationale de l'Eclairage) 1931 chromaticity diagram, shows an example chromaticity of W primary color for use as a white dot along with three primary RGB colors. Here, the chromaticity of this W primary color need not necessarily correspond to the reference white color of a display.
FIG. 4 shows a method of converting an RGB input signal which can display reference white color of a display when R=1, G=1, and B=1, into an RGBW image signal.
First, when the W primary color does not correspond to the reference white color of the display, the following calculation is applied to an input RGB signal for normalization to the W primary color.
                              (                                                    Rn                                                                    Gn                                                                    Bn                                              )                =                              (                                                            a                                                  0                                                  0                                                                              0                                                  b                                                  0                                                                              0                                                  0                                                  c                                                      )                    ×                      (                                                            R                                                                              G                                                                              B                                                      )                                              (        1        )            
In the above expression, R, G, and B represent input signals; Rn, Gn, and Bn represent red, green, and blue signals normalized to W primary color, respectively; and a, b, and c are coefficients which are selected such that brightness and chromaticity equal to W=1 can be obtained when R=1/a, G=1/b, and B=1/c are output.
Then, from Rn, Gn, and Bn, S=F1 (Rn, Gn, Bn) which is used for obtaining values for W dot display, is calculated. Then, based on the calculated value of S, a correction amount F2(S) concerning RGB and a value F3(S) concerning W are calculated.
The correction amount F2(S) is added to each value of RGB to obtain Rn′, Gn′, and Bn′ which are RGB values for achieving RGBW display. On the other hand, the value F3(S) is output unchanged as a W value.
In other word, Rn′, Gn′, Bn′, and W can now be obtained, from S=F1(Rn, Gn, Bn), Rn′=Rn+F2(S), Gn′=Gn+F2(S), Bn′=Bn+F2(S), and W=F3(S) calculated as described above. Similarly, the final process of normalization of Rn′, Bn′, and Gn′ to the reference white color is performed when the W primary color does not correspond to the reference white color of display, by means of the following calculation. This process is performed only as required.
                              (                                                                      R                  ′                                                                                                      G                  ′                                                                                                      B                  ′                                                              )                =                              (                                                                                1.                    ⁢                    a                                                                    0                                                  0                                                                              0                                                                      1                    /                    b                                                                    0                                                                              0                                                  0                                                                      1                    /                    c                                                                        )                    ×                      (                                                                                Rn                    ′                                                                                                                    Gn                    ′                                                                                                                    Bn                    ′                                                                        )                                              (        2        )            
Here, the following are possible examples of basic expressions for calculating S, F2, and F3:S=min(Rn,Gn,Bn)  (1)F2(S)=−S  (2)F3(S)=S  (3)
Specifically, the minimum RGB (normalized Rn, Gn, and Bn) value is set to S, which is used as it as a W value. For obtaining Rn′, Gn′, and Bn′, S is subtracted from each RGB value.
Here, as the pixel color to be displayed approaches an achromatic color, the ratio at which W dot is caused to emit light increases. Accordingly, as the ratio of colors near to achromatic colors increases in an image to be displayed, the ratio of W dots which emit light also increases and power consumption of the panel is lowered compared to when only RGB dots are used.
As normal images are only rarely constituted by just saturated colors, W dots are used in most cases. Consequently, in RGBW display, the power consumption is on average lower than when only RGB dots are used.
In a panel having RGB dots arranged vertically and horizontally, in order to increase the apparent horizontal resolution, the phase of each input signal is made to correspond to the dot position, as shown in FIG. 5. More specifically, the three dot RGB signals are not input at the same timing, but the sampling timing from each image signal is shifted in accordance with the dot position. This results in a display image having higher apparent resolution than when the three RGB dots forming one pixel are driven by input signals having the same phase.
When RGBW dots are used, it is similarly possible to increase the apparent resolution by making the phase of each input signal correspond to the dot position of the panel as shown in FIG. 6. In the above example using the expressions (1) to (3), however, little effect can be achieved because the emission amount of RGB reduces as the ratio of near achromatic color increases in an image. In particular, when the W primary color is the same as the reference white color, no RGB dots are used for displaying a black-and-white image, which results in the resolution of the image corresponding to the number of W dots, as shown in FIG. 7.
Further, when the following expressions are used for F2 and F3, the usage ratio of W dots varies depending on the value of M.F2(S)=−MS  (4)F3(S)=MS  (5)Here, M is a constant which satisfies 0≦M≦1.
In terms of power consumption, it is the most desirable to use M=1. In terms of resolution, however, it is preferable to select the value of M such that all of RGBW emit light.
The present invention advantageously suppresses reduction in the resolution and at the same time reduces power consumption in an OLED display apparatus.